In a fuel cell using hydrogen and oxygen as its fuel, the hydrogen is generated by a reformer in the form of a high-temperature-use heat exchanger. The reformer acts to deliver hydrocarbon such as methanol and steam into a catalyst and applies thereto external heat to generate hydrogen.
Examples of such a heat exchanger for the reformer include one disclosed in Japanese Patent Application Laid-Open Publication No. 2002-80203. This is arranged such that a multiplicity of juxtaposed flat tubes are joined at their respective opposed ends to square tube plates in an gas tight manner to form a core, with the tube plates in a pair being connected to each other by a square-in-section casing. The casing is provided at peripheries around one end and the other end thereof in the longitudinal direction, respectively, with an inlet for a reforming material and an outlet for a reforming gas, with an inlet tank and an outlet tank for a combustion gas being disposed around the peripheries of the tube plates. A bellows-like square-in-section cylinder is interposed between the casing and the tube plates so as to relax a thermal stress occurring between the tubes and the casing.
Due to its square cross-section, however, the square-in-section of bellows-like thermal stress relaxation means tend to suffer from a drawback of being hard to smoothly stretch in its axial direction. In addition, its fabrication is not easy. Mere forming of a cylindrical casing would not allow a fluid to be heated to pass uniformly through parts of the core.
It is therefore the object of the present invention to overcome the above drawbacks.